Is it allowed for taxpayers to transfer receipts (invoices) for accounting purposes to an external company, in accordance with fiscalization requirements in Spain?

it is limited to the SIF. Once the records are duly chained and generated (and signed or sent to the AEAT), the regulation and order do not enter into what is subsequently done with the billing records generated (such as their possible sending to other systems with the idea of using them for other purposes), beyond demanding their conservation, legibility, and accessibility in the future. This means that after the fiscalization is performed, the taxpayers can send the record book for accounting to an external company.